Abdominal aortic aneurysms (AAAs) are a common life-threatening disorder with no therapeutic strategies that effectively blunt growth and progression of the disease. Male sex is a strong risk factor for AAAs. Similarly, AAAs induced by infusion of angiotensin II (AngII) exhibit marked sexual dimorphism with a 4-fold higher prevalence in male compared to female mice. Previous results demonstrated that testosterone exhibits region-specific regulation of angiotensin type 1a receptor (AT1aR) expression in abdominal aortas to promote AngII-induced AAAs. We also demonstrated that exposures of neonatal females to testosterone induced permanent increases in adult susceptibility to AAAs. This model of female androgenization, which mimics surges in testosterone shortly after birth in males, resulted in increased AT1aR expression in abdominal aortas and markedly enhanced AAA susceptibility of adult females. Since males require continued testosterone exposures to exhibit high AAA susceptibility, our results demonstrate that males and females respond differently to testosterone during development. We propose that sex hormones, as well as sex chromosomes, mediate sexual dimorphism of AngII-induced AAAs. The central hypothesis of this proposal is that testosterone effects (developmental and/or adult) at pivotal cell types, in addition to sex chromosome effects, promote region- specific increases in aortic AT1aR expression and AngII-induced AAAs. Aim 1 will define the cell-specific role of androgen receptors in developmental and/or adult effects of testosterone on abdominal aortic AT1aR expression and AngII-induced AAAs. Aim 2 will define the relative contribution of sex hormones versus sex chromosomes in developmental and/or adult effects of testosterone on abdominal aortic AT1aR expression and AngII-induced AAAs. In both aims, approaches will include studies designed to quantify effects on AAA formation versus progression. In addition to identifying mechanisms for sexual dimorphism of AngII-induced AAAs, results from these studies may identify targets, amenable to therapy, that either protect (females) or augment (males) AAA susceptibility.